Conventional evaporators with horizontal tubes traversed by the liquid to be cooled and covered by refrigerant, specifically CCl.sub.2 F.sub.2 (Freon 12), have a maximum heat-transfer coefficient of about 400 W/m.sup.2 K. If the liquid is water, the cooling must not be carried below +4.degree. C. in order to avoid a bursting of the tubes by ice formation. The tubes can be cleaned only after removal of a tightly fitting distributing collar.
When the liquid to be cooled, e.g. water, forms a pool in which upright tubes traversed by the refrigerant are disposed, the minimum temperature can be about +1.degree. C. but the arrangement requires considerably more space. The heat-transfer coefficient is reduced in that case to about 300 W/m.sup.2 K. The same is true when the tubes are replaced by evaporator plates. The cleaning of the outer surfaces of the tubes or the plates is complicated since they are generally immersed to a depth of up to 1.5 m in the liquid with a maximum spacing of about 100 mm.
The best heat-transfer coefficients of about 800W/m.sup.2 K and lowest temperature limit of about +0.2.degree. C. are obtainable with thin-film tube or plate coolers, i.e. horizontal tubes or plates traversed by the refrigerant and covered by a laminar flow of the liquid. For this purpose it is necessary to let the liquid drip onto the tubes or plates through narrow perforations of an overlying distributing plate; these perforations are readily clogged by any solids entrained by the liquid. Cleaning these surfaces again requires considerable work and an interruption of the operation since the flow-confining sidewalls will have to be removed first.
It has already been proposed to provide a heat exchanger with upright heat-transfer tubes along whose inner surfaces a liquid passes downward in the form of a film while the outer tube surfaces are in contact with another fluid. As far as I am aware, however, such systems have not been successful in practice on account of the problems of insuring a laminar liquid flow along the inner tube surfaces. German printed specification (Auslegeschrift) No. 11 64 990, for example, proposes the use of an annular barrier or weir surrounding an upper entrance end of each tube which, however, does not insure the overall and continuous adherence of the liquid flow to the inner tube surfaces. Nor is such adherence guaranteed by an arrangement such as that of Swiss patent No. 600,279 according to which the liquid enters each tube tangentially via several coplanar conduits.
East German (G.D.R.) patent No. 46,722 discloses the provision of a trumpet-shaped distributor pipe which enters the heat-transfer tube from above and has a downwardly diverging bottom part and a flat lower edge separated from the inner tube surface by an annular gap which is interrupted by several spacing ribs. While no dimensions for that gap are given in the patent, its drawing shows the gap width to be about equal to the width of the bottom edge and thus to the wall thickness of the distributor pipe. This arrangement again does not invariably establish the desired type of flow discussed above; I have found, in fact, that the descending liquid tends to adhere to the edge of the pipe so that a significant portion thereof does not even reach the surrounding tube wall but drops free through its interior. With the configuration referred to, even a considerable narrowing of the gap will not bring a significant improvement in that respect.
Furthermore, the presence of a narrow gap between a heat-transfer tube and an inserted distributor body gives rise to an additional problem, namely a clogging of the gap by solids which may be entrained by the liquid to be cooled. In many instances, especially in industrial plants, it is convenient to use river or ground water as that liquid, i.e. as a source of heat to be extracted by the refrigerant and transmitted by the latter via another heat exchanger to a load. The liquid itself, of course, could be used as a coolant for various pieces of machinery after having been brought to a low temperature by heat exchange with the refrigerant.